1. Field of the Invention
The present invention relates to security systems and methods for discouraging thefts or burglaries of powered appliances. More particularly, the present invention relates to security systems that are operable only in response to enablement of a person having knowledge of a predetermined code but which will otherwise disable the appliance. This invention is especially useful for attachment to a post-manufacture powered appliance so that unauthorized use of that appliance requires substantial modification or injury to the appliance.
2. Description of the Prior Art
Thefts and burglaries of relatively portable, powered appliances such as radios, stereos, tape recorders, televisions and the like are a continuing problem since the thief can usually offer an immediately usable appliance to a purchaser of such stolen property. Even with etching of original owner identification on the appliance chassis and/or recordation of appliance serial numbers, recovery of the stolen goods is difficult once a purchaser has acquired the item and put it into use. As a result, interest has remained strong in developing security systems for such powered appliances to reduce the value of the appliance in the hands of anyone other than the owner or those in privy to the owner.
A system for rendering automobile radios unusable when stolen is shown in U.S. Pat. No. 4,734,896 by Soma et al wherein a code from a portable controller must match a stored code in the car radio or an alarm is sounded. A calculator protective device in U.S. Pat. No. 4,103,289 by Kolber responds to failure to enter an initial sequence of characters by disabling the calculator as by continually clearing the registers. It also suggests that it is advisable to place some form of label on the protected device to warn the prospective thief that the calculator is so protected.
A system for responding to failure to enter the correct code by muting an audio amplifier circuit and including a timing circuit operable between retries is shown in U.S. Pat. No. 4,683,462 by Takeda et al. Siebold et al U.S. Pat. No. 4,720,700 shows digital code comparing systems for car radios wherein either the radio must remain in its mount within the car or the code must match a stored code word in a digital memory. Siebold et al also suggest attaching a warning label that the device becomes inoperative if stolen.
Several approaches have pursued adaptation of the microprocessor that already exists in some contemporary electronic appliances to prevent its operation by anyone not having a correct entry code. Typically such systems require entry of a unique digital code which is then compared against a digital code stored in some form of memory associated with the microprocessor. Once a favorable comparison has occurred, a circuit or relay is latched so that primary power remains coupled to the appliance even though the user turns it off and on many times. However, if the primary power is lost as by unplugging the appliance, the system requires reentry and another favorable code word compare before power is again made available to the appliance. Examples of configurations of systems which adapt the existing microprocessor for an appliance to provide power connection controlled security are described in U.S. Pat. No. 4,494,114 by Kaish and U.S. Pat. No. 4,604,708 by Lewis.
Adaptation of appliances having microprocessors incorporated therein is acceptable if the system is designed so as to include such features when the appliance is manufactured. However, once the appliance is built, adaptation of the microprocessor as a security device for that appliance is difficult and prohibitively expensive to the average purchaser. Furthermore, changes to the security code word typically require modifications to the program and/or electronic storage media associated with the microprocessor.